1. Field of the Invention
The present invention generally relates to image processing apparatuses, and more particularly to an image processing apparatus which processes image data so as to display an image corresponding to the processed image data on a screen of a CRT display unit.
2. Prior Art
The conventional display controller sequentially reads image data from a video random access memory (VRAM) in accordance with a scanning operation of a display screen of a display unit, and the display controller displays each dot on the display screen based on the image data. In this case, the VRAM provides a display area corresponding to the display screen and a data area for storing character data and data of a cursor pattern and the like. The access is given to only the display area at a display period and at a period when the image is rewritten on the display screen.
FIG. 1 shows a memory map of a general VRAM. In FIG. 1, e2 designates a display area, and e1 and e3 designate the display areas for storing other data. In this case, the display area e2 consists of display areas e2a and e2b each corresponding to one screen image. In other words, the display area e2 corresponds to two screen images.
In the case where the image data are read from a head address Adx1 of the display area e2a, a number of reading addresses is increased in accordance with the scanning operation of the display screen. Thereafter, when a number of accesses becomes equal to a number of rasters (or a number of rows on the display screen), the reading address returns to the head address Adx1 again so as to display an image depending on contents of the image data stored in the display area e2a on the display screen. In addition, when a scrolling is performed, the reading head address is rewritten by one raster or by one row on the display screen. Furthermore, when a paging is performed, the reading head address is rewritten to a head address Adx2 of the display area e2b. Incidentally, in the case where the display area e2 consists of many display areas each storing one screen image, the process similar to the above-mentioned process will be performed.
Meanwhile, in the case where the reading head address is set to an address Adx3 the value of which is larger than that of an address Adx2, the reading address is increased by predetermined number of rasters (or rows) from the address Adx3. In this case, an access area must advance to the data area e3 next to the display area e2. Hence, the conventional image processing apparatus suffers a disadvantage in that the image processing apparatus must use the data which must not be used for displaying. In addition, a shifted area e2bs is used as the display area. Hence, the conventional apparatus suffers a problem in that the data stored in the data area e3 must be damaged, even when the data stored in the display area are rewritten.